1. Field of the Invention
The present invention generally relates to local area networks employing a token-based ring-of-trees topology and, more particularly, to a method and apparatus for improving the operation of such networks by using rights controlling concentrators.
2. Description of the Prior Art
The interconnection and interoperation of distributed computing devices is an old but continuing challenge. In recent years many of those who have thought about this challenge have concluded that the most effective way to promote progress is to parse the task into relatively independent but connected pieces and develop a corresponding set of standards. Different systems can then communicate if they follow the standards at their corresponding levels; they may rely upon parallel coordination through standards at other levels to complete the communication path. The Open Systems Interconnection (OSI) reference model reflects this view.
The OSI model defines seven layers--from the physical link dealing with mechanical and electrical characteristics of signals to the application layer which provides file transfer and network management services to users. Local area network (LAN) standards within the OSI model are the subject matter and constraints for the present invention. Local area networks are characterized by a topology (e.g., ring or bus) for connecting devices to the LAN's signal transmission medium (e.g., electrical cable or optical fiber) and a protocol (e.g., token-ring or timed-token) for controlling access to the medium by an attached device.
Communication volume over a LAN is limited by the transmission medium and also by the topology and the relevant protocols at both the physical and medium access control levels. "Bandwidth" is a measure of the communication capacity of a LAN, and is commonly given in bits per second. For example, the bandwidth of a coaxial cable ethernet LAN may be 10 million bits per second (Mbps) while a fiber optic distributed data interface (FDDI) LAN using a timed-token protocol may be 100 Mbps.
However, these figures are nominal capacities. Typically, the aggregate communication needs of devices attached to a LAN are an irregular mix of stream and bursty traffic. The LAN's capacity may be under utilized at one time and overloaded at another.
It would be advantageous to have a means wherein each frame that enters and leaves a LAN concentrator essentially unchanged should be expedited through the concentrator quickly with minimal asset utilization. This should be achieved by a means that can selectively control LAN station's "receiving and transmitting rights". Thus, an apparatus is necessary that is capable of separating and controlling a stations' or group of station's "rights of receiving", and its (or their) "rights of transmitting".